The present invention is related in general to cellular communication systems, and, more particularly, to an improved method and system for long code generation in synchronous, multi-chip rate systems.
In synchronous code division multiple access (CDMA) telecommunication systems, a long code generator is initialized by use of the forward link synchronization channel. The long code generator is used to separate users. For example, in the IS-95 system, each user gets a different time shift of the same pseudo noise (PN) sequence, which is the long code. When two synchronized systems are running at multiple and related chip rates, handoffs from one system to the other may be complicated because the state of the long code generator of the new system is not known. For example, CDMA 2000 has a Spreading Rate 1, also known as 1xc3x97, having a chip rate of 1.2288 Mcps, and a Spreading Rate 3, also known as 3xc3x97, having a chip rate of 3.6864 Mcps. If the same long code generator is used in each system, the long code generator in the 3xc3x97 system runs three times as fast as the long code generator in the 1xc3x97 system. This causes a problem when handing off from a 3xc3x97 system to a 1xc3x97 system, and vice versa. Due to the synchronous nature of the two systems, it is difficult to download the long code generator state of the new system to the user during handoff.
In the example discussed above, it is optimum to have a 3xc3x97 long code generator that is derived from the 1xc3x97 long code generator. In this way, once the mobile station is accessing either system, it knows the long code generator state of the other system, and handoffs are simplified. Therefore, a need exists for an improved method and apparatus for long code generation in a synchronous, multi-chip rate system.